Apparatus and method for use in paging mode in wireless communications systems

ABSTRACT

Latency in receiving and detecting paging messages at a wireless terminal is reduced by employing a unique “super” paging time slot format. A paging super time slot includes a plurality of prescribed time slots. Each wireless terminal is associated with cyclically recurring super time slots. A super slot is associated with one or more wireless terminals. A base station always selects the first available time slot in a super time slot to transmit a paging message to a wireless terminal that is associated with the super time slot. Each wireless terminal monitors every time slot in the associated super time slot until either detecting reception of its associated paging message or detecting an empty time slot. In another embodiment of the invention, a unique partially overlapping super time slot format is employed in which a super time slot has at least one time slot common to its adjacent super time slots. This allows wireless terminals associated with adjacent super time slots to share the common at least one time slot. In turn, this results in balanced traffic loads, smoothed traffic fluctuation and reduced congestion.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/756,384, filed Jan. 8, 2001 now U.S. Pat. No. 6,823,191.U.S. patent application Ser. No. 09/774,311 and U.S. patent applicationSer. No. 09/756,370 were filed on Jan. 8, 2001.

TECHNICAL FIELD

This invention relates to wireless communications systems and, moreparticularly, to wireless communications between wireless terminals andbase stations.

BACKGROUND OF THE INVENTION

In wireless communications systems, wireless terminals can maintainconnectivity to a base station without actively communicating with thebase station. Such wireless terminals are referred to as being in apaging mode. In such a system, the wireless terminals (WTs) are paged byassociated base stations (BSs) to initiate communications. In order torealize this, the base stations typically have a channel on the downlinkcalled the paging channel. In prior known paging arrangements, thepaging channel is subdivided into a plurality of paging time slots. Agroup of wireless terminals is typically allocated a paging time slot ofa prescribed periodicity for receiving paging messages from associatedbase stations. As shown in FIG. 1, wireless terminals #1 and #2 areallocated to time slot A, wireless terminals #3 and #4 are allocated totime slot C, and so on. Each mobile unit is expected to monitor forpaging messages from an associated base station during these paging timeslots. Thus, in the example shown in FIG. 1, wireless terminal #1 and #2only monitor time slots A and B, while wireless terminals #3 and #4 onlymonitor time slots C and D, and so on. To this end, the periodicity ofthe paging time slots is made sufficiently long that the each of thewireless terminals can effectively turn off most of its circuitrybetween two of its designated paging time slots and, thus, save energy.This is referred to as the wireless terminal entering a so-called“sleep” mode. The wireless terminal, even though in the sleep mode,still has to keep track of the received paging time slots. The wirelessterminal is caused to “wake up” prior to the arrival of its designatedpaging time slot, tunes to the downlink channel and achieves carrier,timing and frame synchronization. Then, the wireless terminal decodesthe paging time slot and if its identifier is included in the wirelessterminal's designated paging time slot, it knows that the page messageis meant for it. The wireless terminal then takes the appropriate actionindicated in the paging message. If the paging message is not meant forthe wireless terminal, the wireless terminal returns to the “sleep”mode, and monitors the next received paging time slot designated to it.

As seen in FIG. 1, more than one wireless terminal shares the samepaging time slot. The rationale for this is that in general pagingmessages for a particular wireless terminal do not arrive very often,and paging messages for different wireless terminals arrive at mutuallyindependent time slots. Consequently, sharing of the so-called pagingtime slots among several wireless terminals is a more efficient way ofutilizing the paging channel. A problem of sharing of the paging timeslots among several wireless terminals, however, is that latency inreceiving a paging message is increased when several paging messagesarrive simultaneously for the wireless terminals sharing a particularpaging time slot. For example, consider that paging messages arrive intime slot A of FIG. 1 for both wireless terminals #1 and #2, then theassociated base station can only transmit one of the paging messages intime slot A, for example, the message for wireless terminal #1. Then,the base station must wait until time slot B to transmit the pagingmessage for wireless terminal #2. Depending on the arrival rates ofpaging messages and the number of wireless terminals sharing a pagingtime slot, the latency may become undesirably long.

SUMMARY OF THE INVENTION

Problems and limitations of prior wireless communications systems pagingarrangements are overcome, in one embodiment of the invention, byemploying a unique paging “super” time slot format. A paging super timeslot includes a plurality of prescribed time slots. Each wirelessterminal is associated with cyclically recurring super time slots. Asuper slot is associated with one or more wireless terminals.

A base station always selects the first available time slot in a supertime slot to transmit a paging message to a wireless terminal that isassociated with the super time slot. Each wireless terminal monitorsevery time slot in the associated super time slot until either detectingreception of its associated paging message or detecting an empty timeslot.

In another embodiment of the invention, a unique partially overlappingsuper time slot format is employed in which a super time slot has atleast one time slot common to its adjacent super time slots. This allowswireless terminals associated with adjacent super time slots to sharethe common at least one time slot. In turn, this results in balancedtraffic loads, smoothed traffic fluctuation and reduced congestion.

A technical advantage of the invention is that latency in a base stationtransmitting a paging message and, hence, in a wireless terminalreceiving a paging message is significantly reduced.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a prior art time slot format used to transmit pagingmessages in wireless communications systems;

FIG. 2 shows, in simplified block diagram form, a wireless multipleaccess communications system in which the invention may beadvantageously employed;

FIG. 3 shows a unique super time slot format in accordance with theinvention;

FIG. 4 also shows the unique super time slot format useful in describingthe invention;

FIG. 5 shows a unique partially overlapping super time slot format inwhich adjacent super time slots have at least one time slot in common;

FIG. 6 illustrates the unique non-overlapping super time slot format ofFIGS. 3 and 4 and its use in practicing the invention;

FIG. 7 illustrates the unique partially overlapping super time slotformat in which adjacent super time slots have at least one time slot incommon and its use in practicing the invention;

FIG. 8 is a flow chart illustrating steps in the process of a wirelessterminal receiving and responding to a paging message; and

FIG. 9 is a flow chart illustrating the steps in a process of a basestation generating and transmitting paging messages to wirelessterminals.

DETAILED DESCRIPTION

FIG. 2 shows, in simplified block diagram form, a wireless multipleaccess communications system in which the invention may beadvantageously employed. It should be noted that although applicants'unique invention will be described in the context of a mobile wirelesscommunications system, it has equal application to non-mobile, e.g.fixed, wireless communications systems. One such mobile wirelesscommunications system is the Orthogonal Frequency Division Multiplexed(OFDM) based spread spectrum multiple access system.

Specifically, shown in FIG. 2 is a multiple access wirelesscommunications system 200. System 200 includes base station 201including antenna 202 and one or more remote wireless terminals, i.e.,wireless terminals 203-1, 203-2 through 203-Y including associatedantennas 204-1, 204-2 and 204-Y, respectively. Transmission of signalsis from and to base station 201, to and from remote wireless terminals203. All of wireless terminals 203 share the transmission spectrum in adynamic fashion.

In this example, base station 201 includes transmitter 205, receiver 207and controller 206 for transmitting and receiving wireless messages viaantenna 202. Controller 206 is employed to control operation oftransmitter 205 and receiver 207, in accordance with the invention.Similarly, in this example, each of wireless terminals 203-1 through203-Y includes transmitter 208, receiver 210 and controller 209 fortransmitting and receiving wireless messages via antenna 204. Controller209 is employed to control operation of transmitter 208 and receiver210, in accordance with the invention.

Base station 201 transmits paging messages to wireless terminals 203.Typically, wireless terminals 203 when not in use are in a standby modecommonly referred to as a “sleep” mode. In the sleep mode most of thecircuitry in the wireless terminal 203 is turned off in order toconserve energy and, thereby, extend battery life. In order for each ofthe wireless terminals 203 to detect whether there is a paging messageintended for it, the particular wireless terminal 203 must come out ofthe sleep mode, i.e., wake up, and monitor incoming time slots for anassociated paging message. As indicated above, FIG. 1 illustrates aprior time slot format for communication of paging messages between basestation 201 and wireless terminals 203. It is noted that since more thanone wireless terminal 203 can share a time slot, it is possible for morethan one paging message to arrive simultaneously for transmission towireless terminals 203. Since the base station 201 can only transmit onepaging message in a time slot, any additional paging messages must waitfor subsequent time slots to be transmitted. Depending on the number ofsimultaneously arriving paging messages and the number of wirelessterminals 203 sharing a time slot, delay in transmitting a particularpaging message can be significantly long.

FIG. 3 shows a unique super time slot format in accordance with theinvention that significantly reduces latency in the transmission andreception of paging messages. As shown in FIG. 3, a plurality ofprescribed time slots are grouped together into a super time slot, e.g.,time slots A1 and A2 into super time slot #1 and time slots A3 and A4into super time slot #2. These super time slots including appropriatepaging messages are formatted and transmitted by base station 201 towireless terminals 203. In this example, wireless terminals 203-1through 203-4 share super time slot #1. To this end, a paging message isgenerated by base station 201 and inserted into a time slot of a supertime slot associated with the current wireless terminal 203 intended tobe paged. It should be noted that any desired number of time slots maybe grouped into a super time slot and any desired number of super timeslots may be in a cycle of a cyclically repeating super time slotformat.

Note that by grouping time slots into super time slots, the probabilityof congestion at all time slots of any super time slot is significantlyreduced because of statistical multiplexing. Consequently, the latencyin a wireless terminal 203 receiving a paging message is significantlyreduced. However, the wireless terminals 203 are required to monitormore individual time slots, thereby increasing its computational effort.

FIG. 4 also shows the unique super time slot format useful in describingthe invention in reducing the wireless terminals 203 computationaleffort. In order to reduce the computational effort required in wirelessterminals in monitoring the super time slots for paging messages, basestation 201 and wireless terminals 203 follow a prescribed protocol whenemploying the super time slot format shown in FIG. 3. Specifically, basestation 201 always employs the first available time slot to transmitpaging messages to the wireless terminals 203. Each of wirelessterminals 203 monitors its associated super time slot for pagingmessages and abandons monitoring either upon receiving a paging messagedirected to it or upon detecting an empty time slot. An empty time slotis defined as one in which no paging message has been transmitted.

The first available time slot for a paging message is the earliest timeslot in a super slot associated with the intended wireless terminal thatbase station 201 is able to transmit the paging message. By way of anexample, wireless terminals 203-1 through 203-4 share super time slot #1which, in this example, includes time slots A1 and A2. Consider that ina current super time slot, base station 201 has a paging message to besent to an intended wireless terminal 203-1. If in time slot A1, basestation 201 has no other paging messages to be transmitted, base station201, in accordance with the above-noted protocol, transmits the pagingmessage in time slot A1, which is the first available time slot for thecurrent paging message. If there are other paging messages to be sent intime slot A1 as well, base station 201 decides which paging message isto be sent in time slot A1. For example, if base station 201 decides totransmit the current paging message for wireless terminal 203-1 in timeslot A1, then time slot A1 is the first available for the current pagingmessage. Now if base station 201 had decided to transmit another pagingmessage in time slot A1, then time slot A2 may be the first availabletime slot to transport the current paging message for wireless terminal203-1. Furthermore, if base station 201 had decided to transmit otherpaging messages in time slots A1 and A2 in the current super time slot,then the first available time slot for the paging message for theintended wireless terminal 203-1 may be a time slot of the super timeslot associated with wireless terminal 203-1 in the next cycle of theformat. In the above examples of having more than one paging message tobe sent to wireless terminals 203 in a time slot common to the supertime slots associated with the wireless terminals 203, the pagingmessages are contending for the single common time slot. The contentionfor the time slot could be resolved on a random basis. However, this issomewhat undesirable. A better solution is to resolve the contention bya specific fixed process to achieve better system performance, such asreducing latency of transmitting paging messages. In accordance with theinvention, a process is employed which allocates the common time slot tothe paging message intended for the wireless terminal 203 whose supertime slot is earliest among all wireless terminals. The super time slotthat is earliest is described below in relationship to step 905 of FIG.9.

Now assuming that base station 201 has no paging message to betransmitted in super time slot #1, then there is no transmission of amessage in time slot A1 and the time slot is empty. Upon the associatedwireless terminals, e.g., 203-1 through 203-4, detecting that time slotA1 is empty, they stop monitoring time slot A2, thereby reducing thecomputational effort. In accordance with the invention, if base station201 has at least one paging message to be sent for wireless terminalsassociated with a super time slot, base station 201 never has an emptytime slot before a time slot in which paging messages are transported,so that wireless terminals 203-1 through 203-4, in this example, do notmiss any transmitted paging messages because of detecting an empty timeslot that precedes a time slot including a paging message.

It is noted that in the unique super time slot format of FIGS. 3 and 4congestion may occur at a super time slot because of fluctuations in thepaging message traffic. One possible technique for minimizing thiscongestion is to use a super time slot having a larger number of timeslots. Unfortunately, use of more time slots in the super time slotsrequires more computation in order to monitor the super time slot forpaging messages by the wireless terminals 203.

FIG. 5 shows a unique partially overlapping super time slot format inwhich adjacent super time slots have at least one time slot in common.Wireless terminals 203 monitor their associated super time slots todetect paging messages. By way of an example, super time slot #1includes time slots A1 and A2, super time slot #2 includes time slots A2and A3, and so forth, and wireless terminals 203-1 and 203-2 share supertime slot #1, wireless terminals 203-3 and 203-4 share super time slot#2 and so forth. Consequently, in this example, time slot A2 in commonto both super time slot #1 and super time slot #2. Because of thispartially overlapping of time slots by the super time slots the pagingtraffic messages loads on the super time slots can be balanced, therebyreducing the probability of congestion at the super time slots.

FIG. 6 illustrates the unique non-overlapping super time slot format ofFIGS. 3 and 4, and its use in practicing the invention. Assume that basestation 201 has paging messages to be transmitted to wireless terminals203-1, 203-2, 203-3, 203-5 and 203-7. The paging messages for wirelessterminals 203-1, 203-2, 203-5 and 203-7 are transmitted in time slotsA1, A2, A3 and A4, respectively. However, the paging message forwireless terminal 201-3 has to be transmitted in the next occurringsuper time slot #1 because of congestion in the current super time slot#1. Thus, it is seen that the paging message for wireless terminal 203-3is transmitted in the first available time slot in its associated supertime slot #1 that, in this example, is in the next super time slot cycle

FIG. 7 illustrates the unique partially overlapping super time slotformat in which adjacent super time slots have at least one time slot incommon and its use in practicing the invention. Again, assume that basestation 201 has paging messages to be transmitted to wireless terminals203-1, 203-2, 203-3, 203-5 and 203-7. The paging messages for wirelessterminals 203-1, 203-2, 203-3, 203-5 and 203-7 are transmitted in timeslots A1, A2, A3, A4, and A5, respectively. Therefore, it is seen thatthe use of the unique partially overlapping super time slot formatresults in less latency in receiving and detecting the paging messagesin wireless terminals 203-1, 203-2, 203-3, 203-5 and 203-7.

FIG. 8 is a flow chart illustrating steps in the process of a wirelessterminal 203 receiving, detecting and responding to a paging message.The process begins in the power saving mode, i.e., sleep mode, in step801. Thereafter, step 802 tests to determine if a super time slot timerhas timed out. If the test result in step 802 is NO, step 802 isrepeated until it yields a YES result indicating the super time slottimer has timed out. The timing out of the super time slot timerindicates to the associated wireless terminal 203 that has to turn ON,i.e., wake up, in order to monitor its associated super time slot todetect if it has received any paging messages. Then, step 803 causes atime slot index for the current super time slot to be set to K=1. Step804 causes the associated wireless terminal 203 to decode any pagingmessage in time slot K of the super time slot. Step 805 tests todetermine if any decoded paging message is for the particular wirelessterminal 203. If the test result in step 805 is YES, the paging messageis for this particular wireless terminal 203 and step 806 causes theparticular wireless terminal 203 to respond. If the test result in step805 is NO, step 807 tests to determine if time slot K is empty or ifK≧N. N represents the number of time slots in a super slot. If the testresult in step 807 is NO, step 808 causes the wireless terminal 203 toset the time slot index to K=K+1, and then control is returned to step804 and appropriate ones of steps 804 through 808 are iterated untileither step 805 or step 807 yields a YES result. Again, if step 805yields a YES result, step 806 responds accordingly, as described above.If step 807 yields a YES result, step 809 resets the super time slottimer and control is returned to step 801, which causes the wirelessterminal 203 to enter the sleep mode. Thereafter, appropriate ones ofsteps 801 through 809 are iterated, as described above.

FIG. 9 is a flow chart illustrating the steps in a process of a basestation 201 generating and transmitting paging messages to wirelessterminals 203. The process is begun in step 901 by base station 201generating a paging message for a particular intended wireless terminal203. Then, step 902 causes base station 201 to wait for the next supertime slot associated with the intended wireless terminal 203. This isrealized by step 903 testing to determine if a super time slot timer hastimed out. If the test result in step 903 is NO, step 903 is iterateduntil it yields a YES result. The YES result in step 903 indicated thata super time slot has arrived and step 904 causes base station 201 toset a time slot index for the current super time slot to K=1. Then, step905 tests to determine whether there is a paging message for anotherwireless terminal 203 having an associated super time slot prior intime, i.e., earlier, to the super time slot for the particular intendedwireless terminal 203. A super time slot is prior in time, i.e.,earlier, to another super time slot if the last time slot of the formersuper time slot precedes the last time slot of the latter super timeslot. Two super time slots are “contemporary” if their last time slotscompletely overlap. If the test result in step 905 is NO, step 906causes base station 201 to transmit the paging message to the particularintended wireless terminal 203. If the test result in step 905 is YES,step 907 causes base station 201 to transmit a paging message to theother wireless terminal 203 indicated in step 905. (Not shown in FIG. 9,if there is a paging message for another wireless terminal 203 having anassociated super time slot, which is contemporary with the super timeslot for the intended wireless terminal 203, base station 201 may chooseto transmit either of the paging messages on a random basis.)Thereafter, step 908 tests to determine whether time slot index K≧N. Nrepresents the number of time slots in a super slot. If the test resultin step 908 is NO, step 909 causes base station 201 to set the time slotindex to K=K+1 and control is returned to step 905. Thereafter,appropriate ones of steps 905 through 909 are iterated until either step905 yields a NO result or step 908 yields a YES result. If step 905yields a NO result, step 906 causes base station 201 to transmit thepaging message to the particular intended wireless terminal 203, asdescribed above. When step 908 yields a YES result, step 910 causes basestation 201 to rest the super time slot timer and control is returned tostep 902. Then, appropriate ones of steps 902 through 910 are iterated,as described above.

The above-described embodiments are, of course, merely illustrative ofthe principles of the invention. Indeed, numerous other methods orapparatus may be devised by those skilled in the art without departingfrom the spirit and scope of the invention. Moreover, the invention maybe implemented as hardware, as an integrated circuit, via programming ona microprocessor, on a digital signal processor or the like.

What is claimed is:
 1. Apparatus for use in a wireless terminal of awireless communications system to receive paging messages comprising: acontroller for controllably maintaining said wireless terminal in astandby mode of operation; a receiver for receiving a super time slotformat including a plurality of cyclically recurring super time slots totransport paging messages to one or more wireless terminals, whereineach of said plurality of super time slots includes a plurality of timeslots, each time slot intended to transport a wireless terminal pagingmessage; wherein said controller controllably causes said wirelessterminal to enter into a monitor mode for monitoring at least a portionof a received super time slot associated with said wireless terminal fora paging message intended for said wireless terminal and otherwisereturning to said standby mode of operation, wherein the super time slotformat allows the paging message intended for said wireless terminal tobe included in any one of the plurality of time slots of the receivedsuper time slot associated with said wireless terminal, wherein powerconsumption is reduced, wherein said controller causes said wirelessterminal to enter said standby mode responsive to determining that: agiven time slot in said portion of said received super time slotassociated with said wireless terminal does not include any message; andno time slot preceding said given time slot in said portion of saidreceived super time slot associated with said wireless terminal includesa paging message intended for said wireless terminal, and wherein saidportion of said received super time slot comprises two or more timeslots.
 2. The apparatus as defined in claim 1 wherein said controllerdetermines whether said super time slot associated with said wirelessterminal includes a paging message intended for said wireless terminal.3. The apparatus defined in claim 2 wherein said wireless terminalincludes a transmitter and said controller controls said wirelessterminal to transmit a paging response message via said transmitter whenit is determined that said super time slot associated with said wirelessterminal includes a paging message intended for said wireless terminal.4. The apparatus as defined in claim 3 wherein said controller causessaid wireless terminal to enter said standby mode when it is determinedthat said super time slot does not include a paging message intended forsaid wireless terminal.
 5. The apparatus as defined in claim 2 whereinsaid controller is equipped to determine when a time slot in said supertime slot does not include any message and further causes said wirelessterminal to be in said standby mode upon detecting that at least onetime slot does not include a message.
 6. The apparatus as defined inclaim 1 wherein said plurality of time slots is non-overlapping.
 7. Theapparatus as defined in claim 1 wherein adjacent ones of said super timeslots have one or more time slots in common.
 8. A method for use in awireless terminal of a wireless communications system to receive pagingmessages comprising the steps of: maintaining said wireless terminal ina standby mode of operation; receiving a super time slot formatincluding a plurality of cyclically recurring super time slots totransport paging messages to one or more wireless terminals, whereineach of said plurality of super time slots includes a plurality of timeslots, each time slot intended to transport a wireless terminal pagingmessage; and monitoring at least a portion of a received super time slotassociated with said wireless terminal for a paging message intended forsaid wireless terminal and otherwise returning to said standby mode ofoperation, wherein the super time slot format allows the paging messageintended for said wireless terminal to be included in any one of theplurality of time slots of the received super time slot associated withsaid wireless terminal, wherein power consumption is reduced, whereinsaid step of monitoring includes a step of returning to said standbymode of operation responsive to determining that: a given time slot insaid portion of said received super time slot associated with saidwireless terminal does not include any message; and no time slotpreceding said given time slot in said portion of said received supertime slot associated with said wireless terminal includes a pagingmessage intended for said wireless terminal, and wherein said portion ofsaid received super time slot comprises two or more time slots.
 9. Themethod as defined in claim 8 wherein said step of monitoring includes astep of determining whether said super time slot associated with saidwireless terminal includes a paging message intended for said wirelessterminal.
 10. The method as defined in claim 9 further including a stepof transmitting a paging response message when it is determined thatsaid super time slot associated with said wireless terminal includes apaging message intended for said wireless terminal.
 11. The method asdefined in claim 9 further including a step of causing said wirelessterminal to enter said standby mode when it is determined that saidsuper time slot does not include a paging message intended for saidwireless terminal.
 12. The method as defined in claim 9 wherein saidstep of monitoring includes a step of determining when a time slot insaid super time slot does not include any message and further causessaid wireless terminal to be in said standby mode upon detecting that atleast one time slot does not include a message.
 13. The method asdefined in claim 8 wherein said plurality of time slots isnon-overlapping.
 14. The method as defined in claim 8 wherein adjacentones of said super time slots have one or more time slots in common. 15.Apparatus for use in a wireless terminal of a wireless communicationssystem to receive paging messages comprising: means for controllablymaintaining said wireless terminal in a standby mode of operation; meansfor receiving a super time slot format including a plurality ofcyclically recurring super time slots to transport paging messages toone or more wireless terminals, wherein each of said plurality of supertime slots includes a plurality of time slots, each time slot intendedto transport a wireless terminal paging message; means for monitoring atleast a portion of a received super time slot associated with saidwireless terminal for a paging message intended for said wirelessterminal, wherein the super time slot format allows the paging messageintended for said wireless terminal to be included in any one of theplurality of time slots of the received super time slot associated withsaid wireless terminal, and wherein power consumption is reduced; andmeans for returning said wireless terminal to said standby moderesponsive to determining that: a given time slot in said portion ofsaid received super time slot associated with said wireless terminaldoes not include any message; and no time slot preceding said given timeslot in said portion of said received super time slot associated withsaid wireless terminal includes a paging message intended for saidwireless terminal and wherein said portion of said received super timeslot comprises two or more time slots.
 16. The apparatus as defined inclaim 15 further including means for determining whether said super timeslot associated with said wireless terminal includes a paging messageintended for said wireless terminal.
 17. The apparatus as defined inclaim 16 further including means for transmitting a paging responsemessage when it is determined that said super time slot associated withsaid wireless terminal includes a paging message intended for saidwireless terminal.
 18. The apparatus as defined in claim 16 wherein saidmeans for controllably maintaining said wireless terminal in a standbymode of operation causes said wireless terminal to enter said standbymode when it is determined that said super time slot does not include apaging message intended for said wireless terminal.
 19. The apparatus asdefined in claim 15 wherein said plurality of time slots isnon-overlapping.
 20. The apparatus as defined in claim 15 whereinadjacent ones of said super time slots have one or more time slots incommon.